1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, a multifunction peripheral having a function of combining of respective functions of a copier, a printer and a facsimile machine, or such.
2. Description of the Related Art
A fixing unit of melting and fixing a toner (developer) having adhered to a sheet of paper is provided in an image forming apparatus of an electrophotographic type such as a copier, a printer, a facsimile machine, a multifunction peripheral having a function of combining of respective functions of a copier, a printer and a facsimile machine, or such. In the fixing unit, a temperature (fixing temperature) necessary to fix a not-yet-fixed toner image is previously set. However, the necessary fixing temperature may depend on a type of the toner image, a type of the sheet of paper and so forth.
For example, as situations on which the fixing temperature depends, a concentration of a toner having adhered to a sheet of paper and an amount of isolated toner dots having adhered to the sheet of paper may be cited. In a case where a coverage ratio is high and the number of isolated dots is large, the fixing temperature is to be increased to be higher in comparison to a case where a coverage ratio is low and the number of isolated dots is small. Therefore, in an image forming apparatus in the related art, the fixing target temperature is set to meet a severest situation where fixing can be least easily achieved.
However, if fixing is carried out under a fixing condition set to meet a severest situation where fixing can be least easily achieved even in a case where an actual situation is not so sever so that fixing may be easily be achieved, the fixing temperature may be maintained at an unnecessarily high temperature according to the set fixing condition. Therefore, wasteful power consumption may occur in a heating unit, and this may be disadvantageous to the recent demand of saving energy.
Therefore, as will be described below, technologies by which a fixing condition is changed to be suitable between a situation where a toner image can be easily fixed and a situation where a toner image cannot be easily fixed have been proposed.
For example, Patent Document 1 (Japanese Laid-Open Patent Application Publication No. 2009-053421) discloses a technology in which a nip width is adjusted depending on a toner amount of an image to be fixed, and thus, fixing heat quality is optimized.
Patent Document 2 (Japanese Laid-Open Patent Application Publication No. 2006-133580) discloses a technology in which data is analyzed for each pixel, and a fixing temperature is increased in a case of low consumption mode (where toner consumption is reduced) in which an area coverage modulation process is carried out by using a part of carrying out a dither process for black pixels where the number of pixels is large or a part of creating a halftone image by controlling a laser-turning-on period of time in dot units.
Patent Document 3 (Japanese Patent No. 3295273) discloses a technology in which a fixing-starting-up period of time is changed between a character/letter mode and a photograph mode.
Patent Document 4 (Japanese Laid-Open Patent Application Publication No. 2008-185638) discloses a technology in which it is determined whether a colored small pixel is disposed in an isolated manner and a fixing temperature is controlled.
Patent Document 5 (Japanese Laid-Open Patent Application Publication No. 2008-268784) discloses a technology in which in a case where a coverage ratio is high such as a photograph, an optimum fixing temperature is increased in comparison to a case where a coverage ratio is low. Further, Patent Document 5 discloses a technology in which a converge ratio of each of a photograph area and a character/letter area is obtained, and a fixing temperature is controlled to a temperature optimum to each of these areas.
Patent Document 6 (Japanese Laid-Open Patent Application Publication No. 2009-181065) discloses a technology in which in order to control a fixing temperature to an optimum temperature, the fixing temperature is controlled according to image density information based on the number of data dots per certain area.
However, in these related arts, optimum temperature control may be achieved in a case where one recording medium is used to form an image or a case where image forming is carried out in a fixed mode (for example, a photograph mode or a character/letter mode). However, such a manner of control that a fixing temperature is changed for each of plural sheets of paper which are being successively processed is not considered and these related arts may not be suitable for carrying out temperature control for each page during successive processing of plural pages.
More specifically, in the technology of Patent Document 1, it may be very difficult to adjust the nip width for each page during a successive process of plural sheets of paper. It may not be realistic to carry out the control during a successive process of plural sheets of paper in a recent environment where an increase in process speed of an image forming apparatus is being progressed.
Also in the technologies of Patent Documents 2 and 3, a fixing temperature is controlled according to a selected mode, and changing the fixing temperature for each sheet of recording media during a successive process of plural sheets of recording media may not be considered.
In the technology of Patent Document 4, an image is divided into pixels each having a very small area, each pixel is further divided into smaller pixels each having a further smaller area, and fixing control is changed depending on coloring area of each smaller pixel. Therefore, if temperature control is carried out for each sheet of plural sheets of paper during a successive process of the plural sheets of paper, an information amount for each sheet of paper may become an enormous amount. Therefore, it may be difficult to determine how much the fixing temperature can be reduced, and if such a determination is can be carried out, a very large load may be required for the information processing.
Further, Patent Document 5 merely discloses a technology in which a fixing temperature is controlled depending on an image area for a case of fixing an output image onto a recording medium in which image a photograph image and a text image are mixed. Patent Document 5 is silent about a technology of carrying out temperature control for each sheet of paper during a successive process of plural sheets of paper.
In the technology of Patent Document 6, the same as in the technology of Patent Document 4, an information amount required for each sheet of paper may become an enormous amount, so it may be difficult to determine how much the fixing temperature can be reduced, and if such a determination can be carried out, a very large load may be required for the information processing.